244 
COLOUR IN NATURE 
CHAP. 
on the beak, or the skin of the head, as in some 
birds of paradise, is always structural. The naked 
patches of skin in birds indeed exhibit the same 
tendencies with regard to colouring as are visible in 
the feathers. 
Sexual Coloration 
Before giving a detailed account of the colours 
and colouring-matters of birds we may summarise 
some of the general characters of the coloration. 
One of the most marked characters of the group is 
the great prevalence of sexual differences in colour, 
usually though not invariably of such nature that 
the male excels the female in brilliancy, or at least 
in intensity of colour. The colour differences are of 
many kinds. Thus, as in the humming-birds and 
the peacock, the male may display brilliant struc¬ 
tural colour, absent or feebly developed in the 
female; or as in some of the birds of paradise, the 
plumage of the male may be coloured by special 
pigments which are absent in the female. Again, 
as in our own blackbird, the male may, as compared 
with the female, merely exhibit a greater intensity 
of colour. The same fact is illustrated in a curious 
way in the case of the satin bower bird ( Ptilono- 
rhynchus violaceus) ; here the female has a grayish 
somewhat thrush-like plumage exhibiting through¬ 
out faint but distinct metallic tints, so that the 
whole bird has a delicately iridescent appearance. 
The male, on the other hand, is a deep glossy black, 
with a hard metallic lustre—a more specialised, if 
rather less beautiful colour. Another very interest- 
